Hollow acrylate polymer microspheres

ABSTRACT

Hollow, polymeric, acrylate, infusible, inherently tacky, solvent-insoluble, solvent-dispersible, elastomeric pressure-sensitive adhesive microspheres having an average diameter of at least about 1 micrometer. Preferred microspheres are those wherein a majority of the hollow microspheres contain at least one of interior void having a diamter of at least 10% of the diameter of the microsphere. 
     These hollow microspheres are useful as repositionable pressure-sensitive adhesives. The invention also provides pressure-sensitive adhesives consisting essentially of such hollow microspheres. Aqueous suspensions of these microspheres, processes for their preparation, spray repositionable pressure-sensitive adhesive compositions, and microsphere-coated sheet materials are also provided. Surprisingly, hollow microspheres of the invention show reduced or even eliminated adhesive transfer, in comparison with prior art repositionable pressure-sensitive adhesives which are based on solid microspheres.

FIELD OF THE INVENTION

This invention relates to hollow, polymeric, acrylate, infusible,inherently tacky, elastomeric, solvent-dispersible, solvent-insolublemicrospheres, to processes for their preparation, and to their use asrepositionable pressure-sensitive adhesives.

BACKGROUND OF THE INVENTION Description of the Related Art

Solid, inherently tacky, elastomeric microspheres are known in the artto be useful in repositionable pressure-sensitive adhesive applications.As used herein, the term "repositionable" refers to the ability to berepeatedly adhered to and removed from a substrate without substantialloss of adhesion capability. Microsphere-based adhesives are thought toperform well in such applications at least in part due to their"self-cleaning" character, wherein substrate contaminants tend to bepushed aside and trapped between the microspheres as the adhesive isapplied. Upon removal, the adhesive can then still present a relativelyuncontaminated surface for reapplication to the substrate. However,problems with microsphere loss, i.e., microsphere transfer to thesubstrate, and the resultant need for use of a primer or binder havebeen recognized in the art.

Numerous references concern the preparation and/or use of inherentlytacky, elastomeric acrylate polymeric microspheres which are solid innature. Such spheres and their use in aerosol adhesive systems havingrepositionable properties are disclosed in U.S. Pat. No. 3,691,140(Silver). These microspheres are prepared by aqueous suspensionpolymerization of alkyl acrylate monomers and ionic comonomers, e.g.,sodium methacrylate, in the presence of an emulsifier, preferably ananionic emulsifier. The use of a water-soluble, substantiallyoil-insoluble ionic comonomer is critical to preventing coagulation oragglomeration of the microspheres.

U.S. Pat. No. 4,166,152 (Baker et al.) describes solid, inherently tacky(meth)acrylate microspheres which are prepared from non-ionic alkylacrylate or methacrylate monomer(s) in the presence of both anemulsifier and an ionic suspension stabilizer having an interfacialtension sufficient to prevent microsphere agglomeration. Suchmicrospheres are also disclosed in U.S. Pat. Nos. 4,495,318 and4,598,112 (Howard), where the preparative methods involve the use of anonionic emulsifier or a cationic emulsifier. All three patents discloseutility as a "reusable adhesive".

U.S. Pat. No. 4,786,696 (Bohnel) describes a suspension polymerizationprocess for preparing solid, inherently tacky (meth)acrylatemicrospheres which does not require the use of either an ionic comonomeror an ionic suspension stabilizer in order to prevent agglomeration.Rather, the process requires agitation of the vessel charge prior to theinitiation of the reaction sufficient to create a suspension of monomerdroplets having an average monomer droplet size of between about 5 andabout 70 micrometers. In addition to (meth)acrylate monomer, a minorportion of a non-ionic, vinylic comonomer such as, e.g., acrylic acidmay be included to modify the "tacky nature" of the microspheres.

U.S. Pat. No. 3,620,988 (Cohen) discloses a method of preparing"bead-type polymers" which involves the use of a water-insolublepolymeric thickening dispersing agent. The method can be applied toproduces pressure-sensitive adhesives in the form of coatable beadsuspensions, the adhesives comprising a high solidssuspension/dispersion of a lightly crosslinked polymer of a higher alkylacrylate and a tackifier.

U.S. Pat. No. 4,735,837 (Miyasaka et al.) discloses a detachableadhesive sheet having an adhesive layer containing "elasticmicro-balls", wherein the microballs partially protrude from the surfaceof the adhesive layer. The microballs may or may not be tacky. They canbe derived from, e.g., (meth)acrylate monomer and an α-olefiniccarboxylic acid monomer via suspension polymerization in an aqueousmedium. However, no details as to the nature of the surfactantsutilized, etc., are disclosed. The microballs and an adhesive aredispersed in solvent, mixed, and coated, with the ratio of adhesive tomicroballs being from about 1:10 to about 10:1. This ratio is disclosedto be critical in order that all microballs in the final product,including those protruding from the surface, are completely covered withthe adhesive. A range of 1,000 to 150,000 pieces per square centimeteris disclosed as preferred.

DE 3,544,882 Al (Nichiban) describes crosslinked microspheres composedof 90 to 99.5 weight percent of (meth)acrylate ester and 10 to 0.5weight percent of vinyl type monomer, e.g., acrylic acid, having areactive functional group through which crosslinking is achieved byreaction with an oil-soluble crosslinking agent. The microspheres areprepared by dispersing in water a solution (in organic solvent) ofcopolymer prepared by known methods such as solution, bulk, emulsion, orsuspension polymerization. (However, the reference notes that in caseswhere emulsion or suspension polymerization is used with water as adispersion medium, it is not necessary to make a new aqueousdispersion.) When tacky, the spheres are said to be useful in spray orcoated sheet form as "removable adhesive". The stated purpose of theinvention is to provide microspheres having a uniform particle size, butit is also stated that the microspheres may contain other monomers suchas vinyl acetate, styrene, acrylonitrile, methacrylonitrile, etc.,"...to prevent partial transfer of the adhesive when the carrier(backing) is pulled away from the substrate...".

U.S. Pat. Nos. 4,645,783 and 4,656,218 (Kinoshita) disclose a"repeatedly usable and releasable sheet" coated with an aqueoussuspension of microspheres obtained by aqueous suspension polymerization(in the presence of a protective colloid comprising casein as a mainingredient) of one or more alkyl(meth)acrylate esters, one or moreα-monoolefin carboxylic acids, and one or more other vinyl monomers. Themicrospheres are preferably interspersed with finer polymer particlesprepared by polymerization of one or more vinyl monomers in an aqueousmedium. These fine polymer particles are said to be "... effective inimproving the anchorage to the adherend and the adhesion to thesubstrate after the aqueous suspension prepared in accordance with thepresent invention is applied to the substrate".

U.S. Pat. No. 3,857,731 (Merrill et al.) and EP 209337 (Smith &McLaurin) both address problems with microsphere adhesive transfer. Theformer discloses sheets coated with the tacky elastomeric copolymermicrospheres of the Silver patent and a binder material which providessockets in which the microspheres are held by predominately mechanicalforces. The latter states that microsphere adhesives could be put tomore demanding applications if it were not for the drawback of adhesivetransfer. Tacky, elastomeric microspheres are then described which havea composition formed from non-ionic monomers alone or together with aproportion of ionic comonomers. The microspheres further comprise anadhesion promoting monomer having functionality which remains unreactedduring polymerization of the monomers and is available for subsequentlybinding the microspheres through electrostatic interaction or chemicalbonding to a substrate or binder-coated substrate. Preferably, themicrospheres are derived from at least one alkyl acrylate ormethacrylate ester.

In view of the foregoing, it is an object of this invention to reduce oreliminate problems with microsphere adhesive transfer without the needfor a separate binder material or for inclusion of an additionaladhesion-promoting monomer.

It is a further object of this invention to provide an elastomericmicrosphere-based, repositionable pressure-sensitive adhesive whichexhibits greater shear adhesion for a given coating weight of adhesive.Thus, the adhesive can support heavier objects.

It is a further object of this invention to provide an elastomericmicrosphere-based, repositionable pressure-sensitive adhesive whichexhibits greater peel adhesion for a given coating weight of adhesive.This yields a greater amount of tack for an equal weight ofmicrospheres.

It has now been discovered that these objects, and others, which willbecome apparent from the following discussion may be achieved bypreparing microspheres which, in addition to being inherently tacky,elastomeric, infusible, solvent-insoluble, and solvent-dispersible, arealso hollow.

SUMMARY OF THE INVENTION

This invention provides hollow, polymeric, acrylate, inherently tacky,infusible, solvent-insoluble, solvent-dispersible, elastomericpressure-sensitive adhesive microspheres having diameters of at leastabout one micrometer. Preferred hollow microspheres contain one or moreinterior voids having diameters at least 10% of the of the hollowmicrospheres. These microspheres are useful as repositionablepressure-sensitive adhesives.

The invention also provides pressure-sensitive adhesives based on thehollow microspheres, aqueous suspensions of these microspheres,processes for their preparation, spray repositionable pressure-sensitiveadhesive compositions, and pressure-sensitive adhesive-coated sheetmaterials.

Surprisingly, pressure-sensitive adhesives based on hollow microspheresof the invention show reduced or even eliminated adhesive transfer, incomparison with prior art repositionable pressure-sensitive adhesiveswhich are based on solid microspheres. The hollow microspheres of thisinvention are, in effect, "self-priming" and, thus, require neitherseparate primer or binder material nor an additional, adhesion-promotingmonomer. It has also been discovered that, relative to prior artsystems, greater shear and peel adhesion for a given coating weight ofadhesive can be achieved by using hollow microspheres.

This invention also provides a pressure-sensitive adhesive consistingessentially of these hollow microspheres. More specifically, thepressure-sensitive adhesive consists essentially of hollow, polymeric,acrylate, inherently tacky, infusible, solvent-insoluble,solvent-dispersible, elastomeric microspheres comprising:

a) at least about 85 parts by weight of at least one alkyl acrylate oralkyl methacrylate ester; and

b) up to about 15 parts by weight of at least one polar monomer, amajority of the microspheres having one or more interior voids having adiameter of at least about 10% of the diameter of the microsphere.

Aqueous suspensions of these hollow microspheres may be prepared by atwo-step emulsification process comprising the steps of:

a) forming a water-in-oil emulsion of an aqueous solution of polarmonomer(s) in oil phase monomer(s);

b) forming a water-in-oil-in-water emulsion by dispersing thewater-in-oil emulsion into an aqueous phase; and

c) initiating polymerization preferably by application of heat (orradiation).

Aqueous suspensions of hollow microspheres which contain moderatelyionized polar monomer(s) may also be prepared by a simpler ("one-step")emulsification process comprising aqueous suspension polymerization ofat least one alkyl acrylate or alkyl methacrylate ester monomer and atleast one non-ionic polar monomer in the presence of at least oneemulsifier which is capable of producing a water-in-oil emulsion insidethe droplets, as defined below, which is substantially stable duringemulsification and polymerization. Both methods produce an aqueoussuspension of monomer droplets which upon polymerization becomemicrospheres, a majority of which have at least one interior cavitythat, upon drying, becomes a void.

The following terms have these meanings as used herein:

1. The term "droplet" means the liquid stage of the microspheres priorto the completion of polymerization.

2. The term "cavity" means a space within the walls of a droplet ormicrosphere when still in the suspension or dispersion medium prior todrying, and thus containing whatever medium was used.

3. The term "void" means an empty space completely within the walls of apolymerized microsphere.

4. The term "hollow" means containing at least one void or cavity.

All percents, parts, and ratios described herein are by weight unlessspecifically stated otherwise.

DETAILED DESCRIPTION OF THE INVENTION

Alkyl acrylate or methacrylate monomers useful in preparing the hollowmicrospheres and pressure-sensitive adhesives of this invention arethose monofunctional unsaturated acrylate or methacrylate esters ofnon-tertiary alkyl alcohols, the alkyl groups of which have from 4 toabout 14 carbon atoms. Such acrylates are oleophilic, wateremulsifiable, have restricted water solubility, and as homopolymers,generally have glass transition temperatures below about -20° C.Included within this class of monomers are, for example, isooctylacrylate, 4-methyl-2-pentyl acrylate, 2-methylbutyl acrylate, isoamylacrylate, sec-butyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate,isodecyl methacrylate, isononyl acrylate, isodecyl acrylate, and thelike, singly or in mixtures.

Preferred acrylates include isooctyl acrylate, isononyl acrylate,isoamyl acrylate, isodecyl acrylate, 2-ethylhexyl acrylate, n-butylacrylate, sec-butyl acrylate, and mixtures thereof. Acrylate ormethacrylate or other vinyl monomers which, as homopolymers, have glasstransition temperatures higher than about -20° C., e.g., tert-butylacrylate, isobornyl acrylate, butyl methacrylate, vinyl acetate, N-vinylpyrrolidone, acrylamide, and the like, may be utilized in conjunctionwith one or more of the acrylate or methacrylate monomers provided thatthe glass transition temperature of the resultant polymer is below about-20° C.

Polar monomers suitable for copolymerization with the acrylate ormethacrylate monomers are those polar monomers which are both somewhatoil-soluble and watersoluble, resulting in a distribution of the polarmonomer between the aqueous and the oil phases.

Representative examples of suitable polar monomers include moderatelyionized polar monomers such as acrylic acid, methacrylic acid, itaconicacid, crotonic acid, maleic acid, fumaric acid, sulfoethyl methacrylate,and ionic monomers such as sodium methacryate, ammonium acrylate, sodiumacrylate, trimethylamine p-vinyl benzimide,4,4,9-trimethyl-4-azonia-7-oxo-8-oxa-dec-9-ene-1-sulphonate,N,N-dimethyl-N-(β-methacryloxyethyloxy-ethyl) ammonium propionatebetaine, trimethylamine methacrylimide,1,1-dimethyl-1-(2,3-dihydroxypropyl)amine methacrylimide, and the like.Preferred polar monomers are monoolefinic mono- and dicarboxylic acids,salts thereof and mixtures thereof.

The hollow microspheres of this invention and the pressure-sensitiveadhesives made therefrom comprise at least about 85 parts by weight ofat least one alkyl acrylate or alkyl methacrylate ester andcorrespondingly, up to about 15 parts by weight of one or more polarmonomers. Preferably, at least one polar monomer is included in thecomposition, but hollow microspheres may also be prepared using acrylateor methacrylate monomer(s) alone or in combination only with other vinylmonomers, e.g., vinyl acetate. However, when methacrylate monomer aloneis utilized, a crosslinking agent, infra, must be included. For mostpolar monomers, incorporation of from about 1 part to about 10 parts byweight is preferred, as this ratio provides hollow microspheres withbalanced pressure-sensitive adhesive properties.

Aqueous suspensions of the hollow microspheres may be prepared by a"two-step" emulsification process which first involves forming awater-in-oil emulsion of an aqueous solution of polar monomers in oilphase monomer, i.e., at least one acrylate or methacrylate ester, usingan emulsifier having a low hydrophilic-lipophilic balance (HLB) value.Where it is desirable not to include a polar monomer, water may be mixeddirectly with the oil phase monomer, i.e., acrylate or methacrylateester, and emulsifier to form the water-in-oil emulsion. Suitableemulsifiers are those having an HLB value below about 7, preferably inthe range of about 2 to about 7. Examples of such emulsifiers includesorbitan monoleate, sorbitan trioleate, and ethoxylated oleyl alcoholsuch as Brij™ 93, available from Atlas Chemical Industries, Inc. Thus,in this first step, oil phase monomer(s), emulsifier, a free radicalinitiator, and, optionally, a crosslinking monomer or monomers asdefined below are combined, and an aqueous solution of all or a portionof the polar monomer(s) is agitated and poured into the oil phasemixture to form a water-in-oil emulsion. A thickening agent, e.g.,methyl cellulose may also be included in the aqueous phase of thewater-in-oil emulsion. In the second step, a water-in-oil-in-wateremulsion is formed by dispersing the water-in-oil emulsion of the firststep into an aqueous phase containing an emulsifier having an HLB valueabove about 6. The aqueous phase may also contain any portion of thepolar monomer(s) which was not added in step one. Examples of suchemulsifiers include ethoxylated sorbitan monooleate, ethoxylated laurylalcohol, and alkyl sulfates. In both steps, when an emulsifier isutilized, its concentration should be greater than its critical micelleconcentration, which is herein defined as the minimum concentration ofemulsifier necessary for the formation of micelles, i.e., submicroscopicaggregations of emulsifier molecules. Critical micelle concentration isslightly different for each emulsifier, usable concentrations rangingfrom about 1.0×10⁻⁴ to about 3.0 moles/liter. Additional detailconcerning the preparation of water-in-oil-in-water emulsions, i.e.,multiple emulsions, may be found in various literature references, e.g.,Surfactant Systems: Their Chemistry, Pharmacy, & Biology, (D. Attwoodand A. T. Florence, Chapman & Hall Limited, New York, New York, 1983).The final process step of this method of the invention involves theapplication of heat or radiation to initiate polymerization of themonomers. Suitable initiators are those which are normally suitable forfree radical polymerization of acrylate monomers and which areoil-soluble and of very low solubility in water. Examples of suchinitiators include thermally-activated initiators such as azo compounds,hydroperoxides, peroxides, and the like, and photoinitiators such asbenzophenone, benzoin ethyl ether, and 2,2-dimethoxy-2-phenylacetophenone. Use of a water-soluble polymerization initiator causesformation of substantial amounts of latex. The extremely small particlesize of latex particles renders any significant formation of latexundesirable. The initiator is generally used in an amount ranging fromabout 0.01 percent up to about 10 percent by weight of the totalpolymerizable composition, preferably up to about 5 percent.

Aqueous suspensions of hollow microspheres which contain moderatelyionized polar monomer(s) may also be prepared by a "one-step"emulsification process comprising aqueous suspension polymerization ofat least one alkyl acrylate or alkyl methacrylate ester monomer and atleast one moderately ionized polar monomer in the presence of at leastone emulsifier capable of producing a water-in-oil emulsion inside thedroplets which is substantially stable during emulsification andpolymerization. As in the two-step emulsification process, theemulsifier is utilized in concentrations greater than its criticalmicelle concentration. In general, high HLB emulsifiers are required,i.e., emulsifiers having an HLB value of at least about 25, will producestable cavity-containing droplets during the polymerization, and aresuitable for use in this one-step process. Examples of such emulsifiersinclude alkylarylether sulfates such as sodium alkylarylether sulfate,e.g., Triton™ W/30, available from Rohm and Haas, alkylarylpolyethersulfates such as alkylarylpoly(ethylene oxide) sulfates, preferablythose having up to about 4 ethyleneoxy repeat units, and alkyl sulfatessuch as sodium lauryl sulfate, ammonium lauryl sulfate, triethanolaminelauryl sulfate, and sodium hexadecyl sulfate, alkyl ether sulfates suchas ammonium lauryl ether sulfate, and alkylpolyether sulfate such asalkyl poly(ethylene oxide) sulfates, preferably those having up to about4 ethyleneoxy units. Alkyl sulfates, alkyl ether sulfates, andalkylarylether sulfates are preferred as they provide a maximum voidvolume per microsphere for a minimum amount of surfactant. Polymericstabilizers may also be present but are not necessary.

The composition may also contain a crosslinking agent such as amultifunctional (meth)acrylate, e.g., butanediol diacrylate orhexanediol diacrylate, or other multifunctional crosslinker such asdivinylbenzene. When used, crosslinker(s) is (are) added at a level ofup to about 1.0 percent, preferably up to about 0.5 percent, of thetotal polymerizable composition.

The hollow microspheres of the invention are normally tacky,elastomeric, insoluble but swellable in organic solvents, and small,typically having diameters of at least 1 micrometer, preferably in therange of about 1 to about 250 micrometers. The voids typically range insize up to about 100 micrometers or larger. The majority of the hollowmicrospheres prepared by the methods of this invention contain at leastone void with a void diameter which is at least about 10% of thediameter of the microsphere, preferably at least about 20%, morepreferably, at least about 30%.

Following polymerization, an aqueous suspension of the hollowmicrospheres is obtained which is stable to agglomeration or coagulationunder room temperature conditions. The suspension may have non-volatilesolids contents of from about 10 to about 50 percent by weight. Uponprolonged standing, the suspension separates into two phases, one phasebeing aqueous and substantially free of polymer, the other phase beingan aqueous suspension of microspheres having at least one cavity, which,upon drying, becomes a void. Both phases may contain a minor portion ofsmall latex particles. Decantation of the microsphere-rich phaseprovides an aqueous suspension having a non-volatile solids content onthe order of about 40-50 percent which, if shaken with water, willreadily redisperse. If desired, the aqueous suspension of hollowmicrospheres may be utilized immediately following polymerization toprovide inherently tacky pressure-sensitive adhesive coatings. Thesuspension may be coated on suitable flexible or inflexible backingmaterials by conventional coating techniques such as knife coating orMeyer bar coating or use of an extrusion die.

Alternatively, the aqueous suspension may be coagulated with polarorganic solvents such as methanol, with ionic emulsifiers having acharge opposite to that of the emulsifier used in the polymerizationprocess, or with saturated salt solutions, or the like, followed bywashing and drying. The dried hollow microspheres, with sufficientagitation, will readily disperse in common organic liquids such as ethylacetate, tetrahydrofuran, heptane, 2-butanone, benzene, cyclohexane, andesters, although it is not possible to resuspend them in water. Solventdispersions of the hollow microspheres may also be coated on suitablebacking materials by conventional coating techniques, as described abovefor aqueous suspensions.

Suitable backing materials for the aqueous or solvent based coatingsinclude paper, plastic films, cellulose acetate, ethyl cellulose, wovenor nonwoven fabric formed of synthetic or natural materials, metal,metallized polymeric film, ceramic sheet material, and the like. Primersor binders may be used, but they are not required.

Suspensions or dispersions of the hollow microspheres in a liquidmedium, e.g., water or an organic liquid as described above, may besprayed by conventional techniques without cobwebbing or may beincorporated in aerosol containers with suitable propellants such asalkanes, alkenes, or chlorofluorocarbons, e.g., Freons™. The hollowmicrospheres of the invention provide a repositionablepressure-sensitive adhesive, i.e., a pressure-sensitive adhesive havinga degree of adhesion which permits separation, repositioning, andrebonding.

Useful aerosol formulae have a solids content of from about 5% to about20%, preferably from about 10% to about 16%.

The pressure-sensitive adhesive properties of the hollow microspheresmay be altered by addition of tackifying resin and/or plasticizer.Preferred tackifiers for use herein include hydrogenated rosin esterscommercially available from companies such as Hercules Inc., under suchtrade names as Foral™, and Pentalyn™. Individual tackifiers includeForal™ 65, Foral™ 85, and Foral™ 105. Other useful tackifiers includethose based on t-butyl styrene. Useful plasticizers include dioctylphthalate, 2-ethyl hexyl phosphate, tricresyl phosphate,and the like.

It is also within the scope of this invention to include various othercomponents, such as pigments, fillers, stabilizers, or various polymericadditives.

The pressure-sensitive adhesives of the invention have been found toshow little or no microsphere transfer, thereby reducing or eveneliminating the transfer problems disclosed by the prior art. Thesepressure-sensitive adhesives also provide greater peel and shearadhesion for a given coating weight than do prior art repositionablepressure-sensitive adhesives which are solid microsphere-based.

These and other aspects of the invention are illustrated by thefollowing examples which should not be viewed as limiting in scope.

TEST METHODS Microsphere Transfer

An area of coated sheet material was marked and observed using anoptical microscope. The number of microspheres within the area werecounted and this number designated "Y". The marked area of the coatedsheet was then adhered to Kromekote™ paper, a commercially availablepaper for the printing industry, for a few seconds and then removed. Themarked area was again observed with an optical microscope, and thenumber of microspheres remaining in the area were counted and thisnumber designated "Z". Percent microsphere transfer is defined as 100times the ratio of the difference between the number of microspheresinitially present (in the marked area of the coated sheet) after coatingand the number of microspheres remaining in the marked area after eachadhesion and removal from the paper substrate (Y - Z) to the number ofmicrospheres initially present in the area just after coating. ##EQU1##

Peel Adhesion

Peel adhesion is the force required to remove a coated flexible sheetmaterial from a test panel measured at a specific angle and rate ofremoval. In the examples, this force is expressed in grams percentimeter (cm) width of coated sheet. The procedure followed is:

A strip 1.27 cm in width of the coated sheet is applied to thehorizontal surface of a clean glass test plate with at least 12.7 linealcm in firm contact. A 2 kg hard rubber roller is used to apply thestrip. The free end of the coated strip is doubled back nearly touchingitself so the angle of removal will be 180° . The free end is attachedto the adhesion tester scale. The glass test plate is clamped in thejaws of a tensile testing machine which is capable of moving the plateaway from the scale at a constant rate of 2.3 meters per minute. Thescale reading in grams is recorded as the tape is peeled from the glasssurface. The data is reported as the average of the range of numbersobserved during the test.

Shear Strength

The shear strength is a measure of the cohesiveness or internal strengthof an adhesive. It is based upon the amount of force required to pull anadhesive strip from a standard flat surface in a direction parallel tothe surface to which it has been affixed with a definite pressure. It ismeasured in minutes required to pull a standard area of adhesive coatedsheet material from a stainless steel test panel under stress of aconstant, standard load.

The tests were conducted on strips of coated sheet material applied to astainless steel panel such that a 1.27 cm by 1.27 cm portion of eachstrip was in firm contact with the panel with one end portion of thetape being free. The panel with the coated strip attached was held in arack such that the panel formed an angle of 178° with the extended tapefree end which was tensioned by application of a force of 200 gramsapplied as a hanging weight from the free end of the coated strip. The2° less than 180° is used to negate any peel forces, thus insuring thatonly the shear forces are measured, in an attempt to more accuratelydetermine the holding power of the tape being tested. The time elapsedfor each coated film to separate from the test panel was recorded as theshear strength.

ADHESIVE-SOLVENT EVALUATIONS Sprayability

The composition to be tested is sprayed out of an aerosol can ontoaluminum foil at 22° C and an immediate visual evaluation is made.Sprayability is considered "good" when there is a wide pattern of finelyatomized spray with no streaming. Sprayability is "fair" when the sprayis coarse with a narrower pattern and occasional globs are present inthe spray.

Spray Adhesive Transfer

A light coat of the composition is sprayed at 22° C. onto standard whitecopier paper. At a set time after application, the adhesive side of thepaper is briefly contacted with an acetone-wiped plate glass specimenusing hand pressure. The paper is then peeled off and the glass is heldup to a light source to visually determine the amount of adhesivetransfer from the paper to the glass. For an adhesive to be evaluated ashaving no adhesive transfer means that no microspheres are seen on thesubstrate when viewed with the naked eye.

Soak-In on Paper

A light coat of the adhesive composition is sprayed onto standard whitecopy paper and the opposite side of the paper is immediately evaluatedvisually for soak through. "Low" soak-in means that 0-10% of the sprayedarea soaks through to the reverse side of the paper, "moderate" soak-inequals about 50% soaks through, "high" soak-in equals 90% or highersoak-in.

EXAMPLES EXAMPLE 1

In a one-liter resin reactor equipped with mechanical stirrer,condenser, and inlet-outlet lines for vacuum and argon, 450 grams ofdeionized water, 141 grams of isooctyl acrylate, 0.04 gram of1,4-butanedioldiacrylate, 9.0 grams of acrylic acid and 0.5 gram ofbenzoyl peroxide were charged. Vacuum was applied to evacuate thereactor atmosphere, and the reactor was then purged with argon. Theagitation was set to 400 rpm and when the initiator had dissolved, 1.5grams of ammonium lauryl sulfate (Standapol™ A, Henkel AG) were added.The temperature of the reactor was raised to 60° C. and maintained atsuch temperature for 22 hours. An argon purge was maintained during thepolymerization. After the 22-hour period, the suspension was allowed tocool to room temperature. The reactor was then emptied and thesuspension filtered. Optical microscopy revealed hollow microspheresfrom about 4 to about 90 micrometers in diameter suspended in water. Themajority of the microspheres contained a central cavity having a cavitydiameter of at least 30% of the diameter of the microsphere. Afterdrying in a vacuum oven, the microspheres were microtomed. Scanningelectron microscopy also showed that the microspheres contained largecentral voids having diameters of from about 2 to about 65 micrometers.

EXAMPLES 2-11

These examples illustrate the use of different polar monomers andinitiators to prepare hollow, tacky elastomeric microspheres using thegeneral equipment and one-step emulsification technique outlined inExample 1. Details of the compositions are listed in Table I. In allcases 1.5 grams of ammonium lauryl sulfate ($tandapol™ A, Henkel AG)were used. The reactor temperature was 60° C. in all the followingexamples except in Examples 6 and 11 where 50° C. was used.

                  TABLE I                                                         ______________________________________                                                                      Micro-                                                 Isooctyl               sphere  Cavity                                  Example                                                                              acrylate Polar         Diameter                                                                              Diameter                                Number (g)      Monomer       (μm) (μm)                                 ______________________________________                                        2      141      9 g    acrylic acid                                                                           4-70    2-35                                  3      147      3 g    acrylic acid                                                                           4-70    2-35                                  4      135      15 g   acrylic acid                                                                           7-70    5-30                                  5      148.5    1.5 g  methacrylic                                                                            3-50    2-30                                                         acid                                                   6      141      6 g    itaconic acid                                                                          5-50    2-30                                  7      147      3 g    sulfoethyl-                                                                            4-55    1-45                                                         methacrylate                                           8      141      9 g    crotonic acid                                                                          4-50    2-35                                  9      147      3 g    fumaric acid                                                                           5-80    2-50                                  10     148      1.5 g  maleic acid                                                                            4-55    5-30                                  11     141      6 g    itaconic acid                                                                          3-80    2-30                                  ______________________________________                                         The initiator used in all cases was 0.5 grams of benzoyl peroxide with th     exception of Example No. 11 for which the initiator was 0.5 grams of          lauryl peroxide.                                                         

EXAMPLES 12 to 17

These examples illustrate the use of various alkyl (meth)acrylate estermonomers (see Table II) for the preparation of hollow, tacky,elastomeric microspheres using acrylic acid as the polar monomer and 1.5grams of ammonium lauryl sulfate as the surfactant. The polymerizationequipment and polymerization technique used were those described inExample 1.

                  TABLE II                                                        ______________________________________                                                                 Acrylic   Initiator                                  Example                  acid      (Benzoyl                                   Number Alkyl methacrylate                                                                              (g)       Peroxide)                                  ______________________________________                                        12     141 g   2-ethyl-hexyl 9.0     0.5 g                                                   acrylate                                                       13     144 g   n-butyl acrylate                                                                            6.0     0.5 g                                    16     144 g   isononyl acrylate                                                                           6.0     0.5 g                                    15     135 g   isooctyl acrylate                                                                           3.0     0.5 g                                           12 g    methyl methacrylate                                            16     141 g   lauryl acrylate                                                                             9.0     0.5 g                                    17     127.5 g isooctyl acrylate                                                                           4.5     0.5 g                                           18 g    vinyl acetate                                                  ______________________________________                                    

EXAMPLES 18 to 24

These examples illustrate the use of various surfactants as well asvarious multifunctional monomers in the preparation of hollow, tackyelastomeric microspheres (see Table III). In all cases 0.5 gram ofbenzoyl peroxide was used except in Example 20 where 0.5 gram of laurylperoxide was substituted. Example 24 illustrates the use of acombination of a nonionic surfactant and an anionic surfactant.

                  TABLE III                                                       ______________________________________                                        Example                                                                       Number Monomers         Surfactant                                            ______________________________________                                        18     141 g   isooctyl acrylate                                                                          1.5 g                                                                              sodium lauryl                                       9.0 g   acrylic acid      sulfate                                      19     141 g   isooctyl acrylate                                                                          1.5 g                                                                              sodium hexadecyl                                    9.0 g   acrylic acid      sulfate                                      20     73.5 g  isononyl acrylate                                                                          1.6 g                                                                              triethanolamine                                     73.5 g  2-ethyl hexyl     lauryl sulfate                                              acrylate                                                              3.0 g   acrylic acid                                                          0.1 g   1,6-hexanediol                                                                diacrylate                                                     21     120 g   isooctyl acrylate                                                                          1.5 g                                                                              ammonium lauryl                                     24 g    isononyl acrylate ether sulfate                                       6.0 g   acrylic acid                                                          0.1 g   1,4-butanediol                                                                diacrylate                                                     22     141 g   isooctyl acrylate                                                                          14 g Triton ™ W/30*                                   9.0 g   acrylic acid                                                   23     141 g   isooctyl acrylate                                                                          1.5 g                                                                              ammonium lauryl                                     9.0 g   acrylic acid      sulfate                                             0.08 g  divinyl benzene                                                24     146 g   isooctyl acrylate                                                                          1.5 g                                                                              ammonium lauryl                                     2.0 g   acrylic acid      sulfate                                             1.0 g   methacrylic acid                                                                           1.0 g                                                                              Siponate ™                                               Y500-70**                                                      ______________________________________                                         *Triton ™ W/30 is a tradename for a 27% aqueous solution (also             containing 27 percent 2propanol), of sodium alkylaryl ether sulfate           available from Rohm and Haas Company.                                         **Siponate ™ Y50070 is a trade name for a 70% aqueous solution of oley     alcohol ethoxylate available from Alcolac Chemical Company.              

EXAMPLES 25C to 27C

These are comparative examples. When a surfactant with a HLB value lessthan about 25 was used in the one-step polymerization process, tacky,elastomeric microspheres having no voids were formed. Polymerizationequipment and polymerization technique used were those described inExample 1. Benzoyl peroxide (0.5 gm) was used as the initiator in allexamples. The compositions are listed in Table IV.

                  TABLE IV                                                        ______________________________________                                        Example                                                                       Number  Monomers         Surfactant                                           ______________________________________                                        25C     141 g  isooctyl acrylate                                                                           1.5 g sodium dodecyl                                     9.0 g  acrylic acid  benzene sulfonate                                26C     141 g  isooctyl acrylate                                                                           1.5 g sodium dodecyl                                     9.0 g  itaconic acid benzene sulfonate                                27C     141 g  isooctyl acrylate                                                                           7.0 g Triton X200*                                       9.0 g  acrylic acid                                                   ______________________________________                                         **Triton ™ X200 is a tradename for a 28 weight percent aqueous             suspension of sodium alkylaryl polyether sulfonate available from Rohm an     Haas Company.                                                            

EXAMPLE 28

The following example illustrates the preparation of hollowpressure-sensitive adhesive microspheres using a two step emulsificationprocess. Six grams of ammonium acrylate were dissolved in 450 grams ofdeionized water. A water-in-oil emulsion was prepared in an Omni™ mixerby stirring 100 grams of the above-mentioned aqueous solution with 144grams of isooctyl acrylate containing 3 grams of Span 80™, sorbitanmonoleate available from ICI Americas, Inc. and 0.5 gram of benzoylperoxide. The remaining ammonium acrylate aqueous solution was placed ina resin reactor similar to that described in Example 1 and 1.5 grams ofammonium lauryl sulfate were added. The agitation was set to 400 rpm.The oil-in-water emulsion prepared previously was added to the reactor.The temperature was increased to 60° C. and maintained for 22 hours.After the 22-hour period, the suspension was allowed to cool to roomtemperature. The reactor was emptied and the suspension filtered.Optical microscopy of the suspension showed microspheres havingdiameters of from about 4 micrometers to about 30 micrometers, amajority containing internal cavities.

EXAMPLE 29

This example illustrates the use of a thickening agent in the aqueousphase of the water-in-oil emulsion for the preparation of hollowmicrospheres by the two-step emulsification process.

The emulsification and polymerization equipment were those described inthe previous example. An aqueous solution of 6 grams of ammoniumacrylate in 450 grams of water was prepared. Two grams of methylcellulose were added and dissolved into 100 grams of this aqueoussolution. A water-in-oil emulsion of the aqueous solution of theammonium acrylate and methyl cellulose in 144 grams of isooctyl acrylatecontaining 5.75 grams of "Span 80" and 0.5 gram of benzoyl peroxide wasprepared as described in Example 29. The water-in-oil emulsion waspoured into the reactor containing the rest of the ammonium acrylateaqueous solution and 1.5 grams of ammonium lauryl sulfate. Thetemperature of the reactor was 60° C. when the water-in-oil-emulsion waspoured, and the agitation was 300 rpm. The reactor was kept at 60° C.for 22 hours. At the end of this period, the suspension was treated asin Example 28. The diameter of the cavity containing microspheres was inthe range of from about 4 to about 40 micrometers.

EXAMPLES 30 to 33

Examples 30 to 33 were executed following the procedure described inExample 27. Materials and conditions used and diameter of the resultingmicrospheres are specified in Table V.

EXAMPLE 34

This example illustrates the use of a photoinitiator.

A solution of 2.5 grams of N-(3-sulfopropyl)-N-methacryloxy ethylN,N-dimethyl ammonium betaine and 150 grams of deionized water wasprepared in a three-neck 200cc Morton flask. Thirty-five cc of theaqueous solution of the betaine were emulsified for 15 minutes into 47.5grams of isooctyl acrylate containing 0.02 gram of butanedioldiacrylate, 4 grams of "Span 80" and 0.125 gram of2,2-dimethoxy-2-phenylacetophenone in an Omni™ mixer to form awater-in-oil-emulsion. The water-in-oil-emulsion was slowly added to thereactor. The reactor contained the rest of the aqueous solution of thebetaine, 1.172 grams of Span 20 and 0.33 gram of Tween 80™, ethoxylatedsorbitan monoleate available from ICI Americas, Inc. while stirring.Fifteen minutes after the completion of the addition of the water-in-oilemulsion, 10 cc of the water-in-oil-in water emulsion were taken fromthe reactor and placed in a transparent rectangular glass cell. The cellwas then irradiated for 10 minutes with ultraviolet light. After theirradiation period, the suspension was recovered and observed in theoptical microscope. Microspheres having diameters of from about 1micrometer to about 20 micrometers were observed. These microspheres hadcavities in their interior. The majority of the microspheres containedat least one void having a void diameter of at least 10% of the diameterof the microsphere.

EXAMPLE 35C

This example illustrates the fact that use of a ionic monomer in theone-step emulsification process yields solid tacky microspheres ratherthan hollow. In a one-liter resin reactor equipped with mechanicalstirrer, condenser, and inlet-outlet lines for vacuum and argon, 450grams of deionized water, 144 grams of isooctyl acrylate, six grams ofammonium acrylate, 0.5 gram of benzoyl peroxide and 1.5 grams ofammonium lauryl sulfate were charged as described in Example 1 underidentical conditions of temperature and agitation. A suspension wasrecovered from the reactor. Optical microscopy showed microspheres offrom about 4 to about 40 micrometers in diameter in the suspension.These microspheres had essentially no internal cavities.

                                      TABLE V                                     __________________________________________________________________________                                    Microsphere                                                                   Diameter                                      Example                                                                            Monomers                                                                              Emulsifier I                                                                           Emulsifier II                                                                           (μm)                                       __________________________________________________________________________    30   150 g isooctyl                                                                        5.5 g                                                                             Span 80                                                                            1.5 g ammonium                                                                          2-40                                               acrylate                                                                 31   150 g 2-ethyl                                                                         5.5 g                                                                             Span 80                                                                            1.5 g ammonium                                                                          2-56                                               hexyl acrylate   lauryl sulfate                                          32   144 g isooctyl                                                                        6.0 g                                                                             Span 80                                                                            15 g Triton W/30                                                                        4-50                                               acrylate                                                                      6 g acrylic acid                                                         33   139.5 g isooctyl                                                                      5.75 g                                                                            Span 80                                                                            1.375 g ammonium                                                                        4-56                                               acrylate         lauryl sulfate                                               10.5 g vinyl                                                                  acetate                                                                  __________________________________________________________________________     All examples contained 0.5 g benzoyl peroxide as the initiator except for     Example 32 which contained 0.5 g lauryl peroxide.                             All examples were prepared at 325 rpm except for Example 32 which was         prepared at 350 rpm.                                                     

EXAMPLE 36

This example compares the adhesive properties of sheet material coatedwith hollow, tacky pressure sensitive adhesive microspheres and withsolid tacky pressure-sensitive adhesive microspheres.

Microspheres prepared in Example 2 (94:6 isooctyl acrylate:acrylic acidhollow microspheres), Example 25C (94:6 isooctyl acrylate:acrylic acidsolid microspheres), and Example 35C (96:4 isooctyl acrylate:ammoniumacrylate non-hollow microspheres) were dispersed in heptane (5%microspheres) and coated with a knife coater on 49 micrometers thickunprimed cellulose acetate film at a coating weight of 4.1 g/m². Thecoated samples were dried at room temperature and conditioned overnightat a constant temperature of 22.2° C. and 50% relative humidity. Thecoated samples were then tested for adhesive transfer, peel adhesion,and shear strength. Test results are shown in Table VI.

                  TABLE VI                                                        ______________________________________                                                  % Transfer                                                                              % Transfer                                                          after one after two                                                           adhesion  adhesions Peel   Shear                                    Microsphere                                                                             and one   and two   Adhesion                                                                             Strength                                 Sample    removal   removals  (g/cm) (Minutes)                                ______________________________________                                        Example 2  6.0      18.6      204    248.0                                    (hollow)                                                                      Example 25C                                                                             48.7      76.9      178.6  59.5                                     (solid)                                                                       Example 35C                                                                             76.4      92.7      153.4  35.0                                     (solid)                                                                       ______________________________________                                    

As can be observed in the test results shown above, the sheet materialcoated with hollow microspheres exhibited such lower adhesive transfer,higher peel adhesion, and higher shear strength than either the sheetmaterial coated with solid microspheres of the same polymer compositionor the sheet material coated with isooctyl acrylate:ammonium acrylatesolid microspheres.

EXAMPLE 37

This example illustrates the use of hollow pressure-sensitive adhesivemicrospheres of the invention in aerosol adhesive systems. Themicrospheres were tested in various solvents for sprayability, papersoak-in and adhesive transfer. As the results in Table VII show, themicrospheres provided good sprayability with low adhesive transfer in awide variety of solvents.

                  TABLE VII                                                       ______________________________________                                                      Spray-   Adhesive Soak-In                                                                              Time                                   Solvent       ability  Transfer On Paper                                                                             (Sec.)*                                ______________________________________                                        Hexane        Good     No       Low    10                                     Isopar B      Good     No       Low    15                                     Heptane       Good     No       Low    20                                     Cyclohexane   Good     No       Low    20                                     Toluene       Good     No       Moderate                                                                             45                                     Methyl Ethyl Ketone                                                                         Fair     No       High   25                                     Ethyl Acetate Good     No       Moderate                                                                             15                                     1,1,1-Trichloroethane                                                                       Fair     No       Low    25                                     Methylene Chloride                                                                          Fair     No       Low    10                                     Tetrahydrofuran                                                                             Fair     No       High   20                                     Pentane       Good     Yes-High Low     300+                                  ______________________________________                                        *Drying time required to obtain no adhesive transfer to glass                 surface                                                                       AEROSOL FORMULATION                                                           ______________________________________                                        Hollow Microspheres   6      grams                                            Solvent               31.5   grams                                            Acetone               12.5   grams                                            A-31                  60     grams                                            ______________________________________                                         The hollow microspheres were as in Example 2 except 1.2% surfactant was       used, and the reaction was run at 65 degrees C.                               The compositions were applied from 6 fluid ounce metal aerosol containers     fitted with 0.123 neoprene gaskets, Seaquist AR83 valves, and Seaquist        81020-18 actuators.                                                           The term "A31" propellant is an industrywide designator for isobutane         propellant having 31 psig at 70 degrees F.                               

What is claimed is:
 1. Hollow, polymeric, acrylate, inherently tacky,infusible, solvent-insoluble, solvent-dispersible, elastomericpressure-sensitive adhesive microspheres having a diameter of at least 1micrometer.
 2. Hollow, polymeric, acrylate, inherently tacky, infusible,solvent-insoluble, solvent-dispersible, elastomeric pressure-sensitiveadhesive microspheres according to claim 1 wherein a majority of saidmicrospheres contain at least one interior void having a diameter atleast about 10% of the diameter of said hollow microspheres.
 3. Hollow,polymeric, acrylate, inherently tacky, infusible, solvent-insoluble,solvent-dispersible, elastomeric pressure-sensitive adhesivemicrospheres according to claim 1 wherein a majority of saidmicrospheres contain at least one interior void having a diameter of atleast about 20% of the diameter of said microspheres.
 4. Hollow,polymeric, acrylate, inherently tacky, infusible, solvent-insoluble,solvent-dispersible, elastomeric pressure-sensitive adhesivemicrospheres according to claim 1 wherein a majority of saidmicrospheres contain at least one interior void having a diameter of atleast about 30% of the diameter of said microspheres.
 5. Hollow,polymeric, acrylate, inherently tacky, infusible, solvent-insoluble,solvent-dispersible, pressure-sensitive microspheres according to claim1 comprising an acrylate polymer having a glass transition temperatureof no greater than about -20° C.
 6. Hollow, polymeric, acrylate,inherently tacky, infusible, solvent-insoluble, solvent-dispersible,pressure-sensitive microspheres according to claim 1 comprising:a) atleast about 85 parts by weight of at least one alkyl acrylate or alkylmethacrylate ester, and b) correspondingly, up to about 15 parts byweight of at least one polar monomer.
 7. The hollow microspheres ofclaim 6 wherein the alkyl acrylate is selected from the group consistingof isooctyl acrylate, 2-ethyl hexyl acrylate, isononyl acrylate, isoamylacrylate, isodecyl acrylate and butyl acrylate.
 8. The hollowmicrospheres of claim 6 wherein the polar monomer is selected from thegroup consisting of acrylic acid, methacrylic acid, itaconic acid,crotonic acid, maleic acid, fumaric acid, and salts thereof.
 9. Aparticulate pressure-sensitive adhesive consisting essentially of thehollow microspheres of claim
 1. 10. A particulate pressure-sensitiveadhesive consisting essentially of the hollow microspheres of claim 6.11. A repositionable spray pressure-sensitive adhesive comprising thehollow microspheres of claim 1 and a liquid medium therefor.
 12. Arepositionable spray pressure-sensitive adhesive according to claim 11further comprising a propellant selected from the group consisting ofalkanes, alkenes, and chlorofluorocarbons.
 13. A sheet material havingcoated on at least a portion thereof the pressure-sensitive adhesive ofclaim
 9. 14. An aqueous suspension comprising the hollow microspheres ofclaim 1.